The present invention relates to a spot welding machine adapted to weld, for example, a covered conductor wire directly to a terminal or the like.
A conductor wire used in an electronic apparatus which is exposed to a high-temperature atmosphere is coated with heat-resistant synthetic resin, such as polyesterimide. The conductor wire of this type is connected to a terminal by means of a welded joint (spot welding) in order that the electrical connection can be maintained for a long period of time even in the high-temperature atmosphere. In welding the conductor wire to the terminal, it is necessary previously to remove an insulating coating film of synthetic resin on the wire. Conventionally, the coating film is removed mechanically or by chemical dissolution using a strongly alkaline solvent. The removal of the insulating coating film, however, requires much time and labor, thus entailing increase in manufacturing cost.
In order to solve this problem, a spot welding machine having a pair of upper electrodes and a lower electrode is conventionally disclosed in Japanese Patent Disclosure No. 52-14553. The respective lower-end side walls of the paired upper electrodes of this welding machine are closely in contact with each other. The upper electrodes are both connected to an AC power source, while the lower electrode and one of the upper electrodes are connected to a DC power source. In welding a conductor wire, coated with an insulating coating film to a terminal or the like by the use of this prior art spot welding machine, a workpiece is first clamped between the respective lower end faces of the upper electrodes and the upper end face of the lower electrode. While monitoring the electrode temperature in the vicinity of the lower ends of the upper electrodes, an AC current is supplied to the upper electrodes for a predetermined period of time. Thereupon, the coating film is heated to be melted, so that the conductor is exposed. Then, after the upper electrodes are fully cooled, a DC current is supplied between the upper and lower electrodes, thereby effecting electric-resistance spot welding of the exposed conductor and the terminal.
According to the prior art spot welding machine constructed in this manner, the heating of the electrodes for the separation of the coating film is controlled by only varing the conduction time while monitoring the temperature of the heated electrodes. The necessary heating time for the separation cannot, therefore, be adjusted according to the thickness of the coating film, for example. Thus, the coating film sometimes cannot be satisfactorily separated due to deterioration of the upper electrodes by electrolytic corrosion or variation of the film thickness. In such a case, the resulting welds may be defective or cannot enjoy good strength, so that it is hard to maintain uniform weld quality. Since the upper electrodes must be fully cooled before the DC welding current is supplied, moreover, the tack period for each welding cycle can be shortened only limitedly. It is therefore impossible to weld a number of workpieces in a short period of time. Thus, the conventional welding machine cannot enjoy high working efficiency.